Fix Heap::AllocateFillerObject() to use double alignment param.

This is called from Runtime, and was dropping the requested alignment
for doubles. Needed for upcoming MIPS CL to remove some more
workarounds for load/store doubles.

BUG=

Review URL: https://codereview.chromium.org/1131123003

Cr-Commit-Position: refs/heads/master@{#28413}

src/heap/heap.cc

@@ -2534,7 +2534,8 @@ AllocationResult Heap::AllocateFillerObject(int size, bool double_align,
                                             AllocationSpace space) {
   HeapObject* obj;
   {
-    AllocationResult allocation = AllocateRaw(size, space, space);
+    AllocationAlignment align = double_align ? kDoubleAligned : kWordAligned;
+    AllocationResult allocation = AllocateRaw(size, space, space, align);
     if (!allocation.To(&obj)) return allocation;
   }
 #ifdef DEBUG